Irrigation Scheduling of Cotton – Peter Cull, PhD Thesis
Irrigated agriculture provides much of the world’s food supply. Over-irrigation results in soil erosion, increases the potential for contamination of surface and ground water through water runoff and leaching, and requires additional chemicals and fertilisers. There is an increasing need for irrigation based farmers to monitor, analyse and develop solutions to enhance crop yield.
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Soil Moisture Monitoring in Onions
Soil And Water Management in a Kiwifruit Orchard
Turfgrass Water Use and Irrigation Scheduling
Irrigation Efficiency at Robinvale
Russet Burbank Potatoes at Ballarat
On the north west coast of Tasmania onions are grown extensively on krasnozem soils. These soils are medium to heavy in clay and are well drained. The topography is undulating and the predominant irrigation system are travelling boom guns.
Many growers are now using a neutron probe for irrigation scheduling. Monitoring the soil water status on a regular basis has identified soil structural problems, causing low crop water use and consequently a poor yield. The following is an example of poor water infiltration following an irrigation which is very typical of a large number of sites monitored on the north west coast, in the 1989/90 onion season.
Monitoring Depth of Water Extraction
It is important for farmers to know the depth of water extraction of onions for irrigation scheduling. On these well drained soils farmers must be careful not to over irrigate and cause water to drain below the root zone.
The root extraction pattern for onions is shown in Figure 1. Following an irrigation of 26mm on the 27/12 the soil profile reading is shown for the 29/12 with water being added to 60cm. Between the 29/12 and 2/1 the onions used water at 20, 30, 40cm at 2.7mm/day. From the 2/1 to the 4/1 the onions continue to use water at 20, 30, and 40cm at 2.2mm/day. Between the 4/1 and the 9/1 the onions daily water use declined to 1.1mm/day indicating the onset of water stress occurred on the 4/1. This has forced the onions to use water at deeper depths including 20, 30, 40, 50, 60 and 80cm (Figure 1). The most important root zone being 0-50cm.
Monitoring Depth of Irrigation Water Infiltration
The crop was then irrigated on the 10/1 with 15mm of water applied. The neutron probe reading on the 10/1 after the irrigation shows the profile is wetter at 20cm and slightly wetter at 30cm representing a total increase of 9mm in the profile (Figure 2).
On the surface, to the farmer, the irrigation looked like it had done a good job. However the 40 and 50 cm readings show that this part of the root zone is still below the refill point and the profile at 60 and 80 cm is right on the refill point after the irrigation on 10/1. This means that when the profile at 20 and 30 cm dries out the crop will rapidly run out of water as the reserves in the rest of the profile are already depleted, catching the farmer by surprise and the crop will be subjected to another period of water stress.
Clearly much of the applied water had not entered the profile as there is no evidence of through drainage, a common problem on these soils. Analysis of the soil moisture readings at 20, 30 and 40cm readings for the whole season confirmed the poor water infiltration.
Figure 3 shows the VSW% for 20, 30 and 40cm for the whole season. The 20cm and 30cm readings show similar trends, wetting with irrigations and rain, and drying with crop water use and water extraction. At 40cm the VSW% has not increased despite irrigation and rainfall. The onions used water at 40cm between the 4/1 and 9/1 (Figure 1) and the VSW% at 40cm dropped below the refill point on the 4/1 until early February (Figure 3). There was no infiltration of irrigation water to 40cm during January to replace water extracted between 4/1 and 9/1. The VSW% at 40cm only increased at the end of the season due to excessive rainfall. There is a general downward trend for each of the 20, 30 and 40cm readings implying poor water infiltration for the whole season.
Monitoring the soil moisture status in onions has identified poor water infiltration of irrigation water. This has subjected the onions to periods of water stress indicated by the low daily water use as measured using the neutron probe. Figure 3 shows clearly that the 40cm reading was below the refill point during the critical stage of onion bulking resulting in a poor yield at the end of the season.
The large droplet size of water from the boom guns is breaking down the soil structure on the surface causing poor infiltration, runoff and erosion. In the following season growers must look carefully at their probe readings following irrigations to identify ineffective irrigations.
Dr Peter Cull
Through regular soil water monitoring of orchard crops using the neutron probe, farmers and consultants have gained valuable insights into the problems associated with frequent wheel tractor activity on orchard soils under frequent sprinkler irrigation. In a wide range of crops such as kiwifruit at Shepparton, grapes at Pokolbin and macadamias at Bundaberg, neutron probe data has shown that dry subsoils, a result of poor infiltration of irrigation water into the root zone, and a small effective root volume caused by mechanical compaction of the inter-row subsoil are major factors determining yield and quality of orchard crops.
Two important questions are often asked by orchardists in relation to soil water management. They are, firstly, what is the effective root volume of my tree or vine and what has been the long term effect of inter row cultural practices on soil structure and plant water use.
The answers to these questions are being provided by neutron probe soil water data collected in the course of regular soil water monitoring. The data is being used increasingly by orchardists to make sound agronomic decisions as to the need for fundamental changes to be made in their orchard management programs. The two major considerations are the need for deep ripping between tree rows to break up hard pans and the effectiveness of the existing sprinkler or drip irrigation system on variable soil types.
In order to determine the effective root volume of a kiwifruit vine in sand soil, five aluminium tubes were inserted along the row at 30 cm intervals from the base of the kiwifruit vine, tube 1 being the closest at 30 cm from the vine and tube 5 the furthest at 150 cm from the vine (Figure 1). The sprinklers were removed from the surrounding area and the soil water content measured as the soil dried out. The soil water content was measured daily at depths of 20, 30, 40, 50, 60, 80, 100 and 120 cm using the neutron probe.
The neutron probe readings taken on 20 January showed the soil water content for Sl five tubes to be essentially the same with only 9 mm difference in soil water content between tube 1, the wettest, and tube 5, the driest at 18 mm below the previously measured full point of 200 mm in the top 0-70 cm which is the main root zone.
The refill point was reached on 24 January as determined by a decrease in daily water use at all tubes. The average daily water use between 20 January and 24 January decreased with distance away from the vine and decreased after 24 January from 7.8 to 3.6 mm/day for tube 1 and from 3.8 mm/day to 3.2 mm/day for tube 5. The refill point occurred at increasing soil water content with distance away from the vine. The depth of soil water extraction decreased from 80 cm at tube 1 to 40 cm at tube 5 at the refill point on 24 January.
Additional data for tubes placed at 30 cm intervals perpendicular to the kiwifruit vine row would provide a three dimensional picture of soil water extraction and hence effective root volume for the kiwifruit vine. The effect of sprinkler pattern and inter row compaction on root distribution may be readily determined from the data.
In the course or regular soil water monitoring, twice weekly measurements of soil water content in kiwifruit on sand soil were made over two summer growing seasons from December 1986 to February 1988 (Figure 2). The first neutron probe reading taken on 16 December showed the profile to be at the full point (180 mm, 0-70 cm). However despite frequent irrigation the soil water content declined rapidly to the refill point.
The soil water extraction pattern during the critical fruit sizing period from December 1986 to April 1987 (Figure 3) shows water use at 80 cm only at the refill point (130 mm on 7 January). By 15 April, however the soil water content was severely depleted at depths greatly exceeding 120 cm. Kiwifruit size during this time was substantially reduced as a result of plant water stress.
The period from 15 April to 15 May was characterised by frequent rainfall supplemented by irrigation which allowed good infiltration of water into the 070 cm layer of the profile as shown by an increase in soil water content from 80 to 150 mm (0-70 cm). The existence of a compaction layer in this very sandy soil is evident from an infiltration “bottle-neck” at 6080 cm (Figure 4). On 15 May no water infiltrated below 80 cm despite quite high soil water content in the 0-70 cm soil layers.
Generally adequate levels of 0-70 cm soil water content were maintained from May to October 1987 as a result of winter rainfall and reduced plant water requirements, despite the dry subsoil which existed, but by November the soil water content was again approaching critically low levels.
The soil was deep ripped to a depth of 60 cm on 30 November resulting in a rapid and substantial increase in subsoil water levels (Figure 5). The soil water content during this second season was maintained at optimum levels during the critical fruit sizing period resulting in much higher yield and quality of kiwifruit.
Neutron probe readings in other orchard situations have revealed similar soil structural problems such as compaction and poor infiltration of water into the subsoil. By monitoring soil water with the neutron probe these conditions may be quantified so that appropriate changes can be made and their consequent effect on soil water recorded. The neutron probe is an effective tool for establishing critical soil water levels which must be attained by bud-burst and maintained through to fruit maturation if high yields of export quality kiwifruit are to be achieved.
Turfgrass managers know their turfgrass on the surface, however below the surface there is a considerable amount of guess-work on the depth of water extraction, and the amount of water available for turfgrass growth. It is important that the turfgrass manager maximise rooting depth so that turf is able to draw moisture and nutrients from a greater portion of the soil profile (Beard, 1985).
Prior to a sporting event many factors need to be considered when deciding to water. By not watering there is a risk that the playing surface will not be suitable for the event. However by watering there is a chance that it will rain and the playing surface will again be unsuitable. Therefore the turfgrass manager needs to know exactly how much water is in the soil profile at each depth so it can be calculated how much water if any, needs to be applied to the turfgrass to maintain the desired quality of turfgrass for the event duration.
Recent government reports have highlighted the growing scarcity and rising costs of water, and the need for more efficient use of water by turfgrass managers for recreational areas. With the ever increasing demand for our finite resource water, efficient water management should be a high priority for turfgrass managers who aim to reduce costs.
To improve the water use efficiency on turfgrass, turfgrass areas should be watered according to the soil moisture status and turfgrass needs. It has been shown that by adjusting irrigations according to soil moisture sensing devices as opposed to a pre-planned routine, it is possible to reduce water consumption by 40-50% without reducing turfgrass quality (Shearman, 1985).
It is often stated that the most efficient way to water turf is to practice deep and infrequent waterings. However there is very little information that quantifies the amount of water, irrigation frequency and the depth of application required, and these form the objectives of this trial outlined below.
Four neutron probe access tubes were installed on the 10th Fairway of the Narrabri Golf Course, Northern N.S.W.. The tubes were installed across the radius of the sprinkler as shown in Figure 1. The fairway consists of kikuyu grass growing in a clay soil and there were no trees within 50 metres of the tubes. The soil water content was measured regularly at depths of 10, 20, 30, 40, 50, 60, 70 and 80cm below the surface using a neutron probe.
How deep did the kikuyu extract water ?
The kikuyu roots have extracted 67 mm of water to a depth of 80 cm between the 30/12/88 and the 30/1/89 (Figure. 2). The soil profile measured 290 mm on the 30th December following a very wet Christmas/New Year period so it was called the full point. On the 30/1/89 the kikuyu reached its refill point, which represents the driest soil water profile prior to the turfgrass quality falling below an acceptable standard for its intended purpose. The depth of turfgrass water use is important because the turfgrass manager needs to know if they are gaining maximum benefit of the stored soil water and nutrients.
How do you determine the Refill Point ?
Regular neutron probe readings were taken during January, February and March 1989, and the 0-90 cm profile water content of Tube 3 is shown in Figure 3. The profile water content decreased as the kikuyu extracted water. The daily water used by the kikuyu is indicated by the slope of the graph. After the irrigations, the kikuyu used water at a greater rate (5 mm/day) than it did as it approached an irrigation (2 mm/day). This decline in daily water use indicates that the kikuyu is finding it harder to extract the available water from the profile. On the 30th January the daily water use dropped below 1 mm/day and it was decided to make this the refill point at 223 mm. This represents a deficit between irrigations of 67 mm. At this stage the quality of the kikuyu was still acceptable according to the local golfers. The fairway was then irrigated until it reached the previous full point.
During the second drying cycle some rain fell. The fairway also received a small irrigation on the 14th February when the profile was at 251mm which increased the profile water content to 262 mm. This irrigation occurred because the very outside of the fairway had reached its refill point. The uneven sprinkler distribution is discussed in the following section. The profile was then allowed to dry to the previous refill point. The daily water use again declined at the refill point, however this time the fairway was not irrigated to see how long it would be before the kikuyu would show visible wilting symptoms. This occurred about four days later as the daily water use was very low. This provided further evidence that the refill point was correct and the turfgrass manager could be confident in watering the fairway according to the refill point.
Dry Sub Soil
An additional trial site was installed on the course. This site was very dry and the kikuyu was brown in colour. On the 28th January a reading was taken which was well below the refill point. The site was then irrigated for four hours and another reading was taken the next day (29th January, Figure 4).
After four hours of watering the profile had been wet to 60cm (Figure 4). The kikuyu responded very quickly to the applied water regaining its green colour. The profile was allowed to dry until the 8th February – the refill point. It was not possible for the kikuyu to extract any water deeper than 60cm because of the dry sub soil which was not wet during the irrigation unlike the kikuyu on the fairway (Figure 2) which had a wet sub soil. Hence this grass wilted much more quickly than on the fairway.
Problems with the Irrigation System
As shown in Figure 1, four neutron probe tubes were installed across the radius of the fairway sprinkler. The profile water contents of Tubes 2, 3 and 4 are shown in Figure 5. On the 30th January Tube 3 reached the refill point as discussed in other sections of this paper. However tube 4 on the same day is shown to be much drier and hence why the kikuyu on the outside of the fairway was severely wilted. Tube 2 on the same day still has some time before it will reach the refill point. Clearly there are large differences in the soil moisture content across the fairway being caused by an uneven wetting pattern of the sprinkler.
Following a period of wet weather during March and April the neutron probe readings showed that all four tubes had returned to the same moisture content. It was also necessary to modify the previous full point 290 mm, set on the 30/12/88 to the full point 315 mm, set on the 12/4/89 (Figure. 2). Most of this additional stored moisture is in the sub soil (Figure. 5).
There exists considerable scope for improvement of water use efficiency on turfgrass areas in Australia. Turfgrass areas should be watered according to the soil moisture status and turfgrass needs rather than on a set routine of once a week for example.
By monitoring the soil moisture status the turfgrass manager will need to spend less time watering and mowing, reduce disease and nutrient leaching, highlight problems with irrigation system design, identify soil management problems such as compaction, conserve water and most importantly reduce costs.
Measuring the soil moisture status allows the turfgrass manager to know…
1. The depth of the root zone.
2. Amount of water needed for an irrigation.
3. Establish a refill point.
4. Measure the effectiveness of rainfall.
5. Daily turfgrass water use.
6. Infiltration rates and drainage.
7. Soil management problems such as compaction.
8. Set an irrigation schedule.
Beard, J.B (1985), A new prospective on root growth. University of California Publication 21404.
Shearman, R.C. (1985) Turfgrass culture and water use. University of California Publication 21405.
Wemen Vineyards at Robinvale, Victoria, are using sprinklers to irrigate wine grapes, with tile drains installed at 1.2 m depth to drain off excess irrigation water. They are recording the number of hours that water was pumped at each irrigation; the Neutron Probe is measuring the amount of water that actually reached the crop root zone at each irrigation; and a record is kept of the volume of water running out of the drains, on a scale from 1 to 10. The pipe has been marked off in 10 equal discharge volumes. The objective was to monitor the efficiency and effectiveness of each irrigation, and to graphically display the results.
The solution was to make four keydata entries, called EFFEC, HOURS, +EFFEC/HOURS and DRAIN on the Edit Site screen (Figure 1). EFFEC is the effective depth of water that reached the soil profile, as measured by the Neutron Probe; HOURS is the pumping time in hours; the ratio of EFFEC to HOURS will be a measure of the irrigation efficiency; and DRAIN is an arbitrary number from 1 to 10 . The actual units of the ratio will depend on pump capacity and area irrigated, but it will immediately give an indication of efficiency.
Each time an irrigation was posted to the readings screen (Figure 2), the pumping time (in hours) was also entered in the Irrigation column, in the Keydata HOURS row. The depth of water that actually reached the soil profile, after adjustment to get a reasonable ProbeDWU, was copied from the first row of the spreadsheet to the Keydata EFFEC row. After updating, the +EFFEC/HOURS was calculated by the Probe software.
By selecting the +EFFEC/HOURS keydata row, and plotting a time graph of selected data, users could quickly see the efficiency of all irrigations in the season. However, in order to actually plot vertical bars for HOURS on the time graph a slight ‘adjustment’ was required. A ‘dummy’ estimate column was inserted immediately after each irrigation, and a value of “0” was inserted in the keydata rows in the (Est) columns before and after the irrigation. In this way, when the HOURS Keydata row was selected, vertical columns with a symbol at the top are drawn, on top of the normal Irrigation bars (Figure 3).
In this example, in 6 hours of pumping 30 mm was applied at a rate of 5 mm/hour, but only 17 mm was effective – a rate of 2.8 mm/hour. The difference is being lost in the pumping system, by evaporation, or by through drainage. An examination of the depth graphs will show the amount of drainage. In this case, pumping should have been for only 3.4 hours (17 mm at 5 mm/hour).
Irrigation scheduling with the neutron probe in the Ballarat area has resulted in significant improvements in irrigation management in recent years. Improved grower awareness of the importance of irrigation for yield and quality has resulted in less excess water use in potato crops, as demonstrated by Marshall and Gowers (1987).
During the 1990/91 season, Russet Burbank potatoes growing on red krasnozem soils were monitored as part of a commercial service offered to all growers. In previous years the service concentrated on when to irrigate and how much water to apply. There was very little interpretation of the neutron probe data to understand how much water was moving through the soil profile and when plant stress may have occurred.
Crops monitored this year were scheduled very well with the irrigation systems available (travelling gun types). There were few occasions when irrigation was considered untimely and there was no evidence of stress due to lack of water.
The season was relatively dry during November and December. The Christmas to New Year period was hot with temperatures above 35oC, followed by 70mm rainfall on the 5th January. The remainder of the season was mild to hot without extremes of temperature except for a hot spell on the 6th and 7th March. There were about 3 weeks of windy conditions in February.
Three aluminium access tubes were placed in the plant row in representative areas of potato fields. Twice weekly neutron probe readings were taken at 20, 30, 40, 50, 60 and 80cm and averaged for the three aluminium tubes.
Significant differences were identified between the amount of water applied to the crop and that measured as crop water use. This may be due to:
1. Run-off from the “hilled” soil in the plant row away from the area of measurement. This may occur, but only partly explains the difference.
2. Through drainage.
The seasonal trend in soil water content for a potato crop at Ballarat is shown in Figure 1. At first glance there appears to be only 2 occasions when excess water was applied causing the line to go above the Full Point.
However, careful analysis of the daily water use (DWU) figures and the soil profile graphs indicate that at 12 of the 20 recorded irrigation and rainfall events, more water was applied than could be used by the crop (Figure 2). In general, this occurred if more than 20mm of irrigation water was applied, resulting in loss of 10-15mm of irrigation water for a typical 30-35 mm irrigation.
A high level of interpretive skill is required to detect through drainage events from soil profile graphs. Small changes (1-2%) in volumetric soil water content may account for large differences between recorded irrigation or rainfall data and crop water use.
Figure 3 shows changes in soil water content (VSW) at each depth in the profile. Following 70mm rainfall on the 5/1, the soil water content was still high (above the Full Point) at 293mm on the 7/1. Between the 7/1 and the 10/1 there was a significant decrease in soil water content at all depths. Measurements of root length in this crop show little root growth below 30cm.
It appears that the large change in soil water content at 40-80cm is due to through drainage and not plant root extraction. Between the 10/1 (283mm) and the 17/1 (272mm) there was no change in soil water content at 60cm and only a very small decrease at 50cm. This is consistent with the pattern of plant water use in March which shows no change in soil water content at 60cm from 1/3 (281mm) to 4/3 (273mm). On 7/3 (264mm), there was plant water use at 60cm as the more developed root system late in March dried the soil deeper than earlier in the season.
Through drainage may occur for up to 5 days following heavy application of water. The water drains from the soil regardless of the soil water content at the time of watering. However when less than 20mm of irrigation water is applied, there is usually no loss of water by through drainage. This suggests that no more than 20mm water should be applied per irrigation, regardless of how dry the soil profile is at the time of irrigation, to avoid leaching of excess water.
These findings have important implications for crop nutrition as applied fertiliser is likely to be leached from the soil profile. This is particularly important for early crop growth, and a strategy of reducing fertiliser application at planting and topping up nutrition as the crop grows may well improve fertiliser use and crop nutrition.